Endovascular prostheses are sometimes used to treat aortic aneurysms. Such treatment includes implanting a stent or stent-graft within the diseased vessel to bypass the anomaly. An aneurysm is a sac formed by the dilation of the wall of the artery. Aneurysms may be congenital, but are usually caused by disease or, occasionally, by trauma. Aortic aneurysms which commonly form between the renal arteries and the iliac arteries are referred to as abdominal aortic aneurysms (“AAAs”). Other aneurysms occur in the aorta, such as thoracic aortic aneurysms (“TAAs”), which may occur in one or more of the descending aorta, the ascending aorta, and the aortic arch.
Endo-Vascular Aneurysm Repair (EVAR) has transformed the practice of treatment of aortic aneurysms from an open surgical approach to a much less invasive surgical approach. The first step of an endovascular intervention usually requires introducing a delivery system into the vasculature of a subject. If the crossing profile, i.e., the external diameter, of the delivery system is 14 Fr or lower (3 Fr=1 millimeter), a true percutaneous approach may be used, because vascular closure devices are available for proper closure of such puncture sites. If the crossing profile at least 15-16 Fr, a vascular cut-down is usually required in advance as a preparatory step to introduction of the delivery system.
Endovascular systems for treatment of supra-renal aneurysms generally require the preparatory step of a vascular cut-down. A cut-down is the localized surgical exposure of blood vessels for accessing the subject's vasculature. For example, most surgical cut-downs used in EVAR procedures are performed in the vicinity of the pubis, exposing the iliac arteries. Surgical cut-downs have related complications and co-morbidities, including difficulty in controlling bleeding at the access site, false aneurysms, and vascular obstruction. It is therefore desirable to use a purely percutaneous approach, instead of a vascular cut-down.
Endovascular stent-grafts for treating the thoracic aorta usually require a 20-22 Fr delivery system, because of the large amount of graft material indicated by the diameter of the aorta above the level of the renal arteries (30-40 mm diameter or more in some subjects). Currently used graft materials are PET (Poly Ethylene Therephtalate) and ePTFE (expanded Poly-Tetra-Fluoro-Ethylene). The thickness and circumferential length of the graft have the most substantial effect on the crossing profile of an endovascular system. The use of thinner graft materials generally reduces long-term durability of the graft material.
“Endoleak” is the persistent flow of blood into the aneurismal sac after implantation of an endovascular prosthesis. The management of some types of endoleak remains controversial, although most can be successfully occluded with surgery, further stent implantation, or embolization. Four types of endoleaks have been defined, based upon their proposed etiology: Type I endoleak, described below; Type II endoleak, characterized by flow into and out of the aneurismal sac from patent branch vessels; Type III endoleak, characterized by flow into the aneurismal sac from separation between components of a modular system; and Type IV endoleak, characterized by egress of blood through the pores in the fabric.
A type I endoleak, which occurs in up to 10 percent of endovascular aortic aneurysm repairs, is due to an incompetent seal at either the proximal or distal attachment sites of the vascular prosthesis, resulting in blood flow at the end of the prosthesis into the aneurismal sac. Etiologies include undersizing of the diameter of the endograft at the attachment site and ineffective attachment to a vessel wall that is heavily calcified or surrounded by thick thrombus. Type I failures have also been found to be caused by a continual expansion of the aneurysm neck (the portion of the aorta extending cephalad or caudad from the aneurysm, and is not dilated). This expansion rate has been estimated to be about one millimeter per year. Because the aneurysm neck expands beyond the natural resting diameter of the prosthesis, one or more passageways are defined about the prosthesis in communication with the aneurismal sac. Additionally, Type I endoleaks may be caused when circular prostheses are implanted in non-circular aortic lumens, which may be caused by irregular vessel formation and/or calcified topography of the lumen of the aorta.
Type I endoleaks may occur immediately after placement of the prosthesis, or may be delayed. A delayed type I endoleak may be seen during follow-up studies if the prosthesis is deployed into a diseased segment of aorta that dilates over time, leading to a breach in the seal at the attachment site.
Type I endoleaks must be repaired as soon as they are discovered, because the aneurismal sac remains exposed to systemic pressure, predisposing to aneurysmal rupture, and spontaneous closure of the leak is rare. If discovered at the time of initial placement, repair may consist of reversal of anticoagulation and reinflation of the deployment balloon for an extended period of time. These leaks may also be repaired with small extension grafts that are placed over the affected end. These methods are usually sufficient to exclude the aneurysm. Conversion to an open surgical repair may be needed in the rare situation in which the leak is refractory to percutaneous treatment.
As can be readily appreciated, even with the successful implantation of an endovascular prosthesis, failures may occur thereafter. It has been found that type I endoleak failures may affect up to 5-10% of all implanted prostheses. Accordingly, there is a clear need for an endovascular prosthesis which can reduce the likelihood of, and ideally eliminate, type I endoleak failures.